This invention relates to electron beam systems and more specifically to an electron beam blanker for use in electron beam columns.
Beam blanking is a required function for electron beam systems used in lithography, testing, metrology, and inspection. In the field of electron beam lithography, the electron beam is switched on and off by deflecting it against an aperture stop (blanking). The deflection is typically accomplished by an electric field between two parallel plates. The plates form a waveguide structure which conducts electromagnetic blanking signals used to define, in part, the pattern to be exposed by effectively switching the beam on and off.
FIG. 1 shows an example of an electron beam column including such a blanker. FIG. 1 is taken from Kelly et al., U.S. Pat. No. 4,445,041, incorporated herein by reference, and uses the same reference numbers as does Kelly. Electrons are provided in the column by a cathode 5 which is a field emission electron source. Cathode 5 is supported above an anode 10 with the anode serving to control and effectively collimate the electron beam. As the electrons move down the column from the anode, they encounter a first lens 15 which serves to focus the beam at the center of a beam blanker 25. Along that path, an alignment deflector/stigmator 20 aligns the beam with the optic axis and stigmates the beam to provide the proper shape before the electrons enter the blanker. Blanker 25 then blanks the beam at the appropriate time to control the exposure when a target 65 at the bottom. A second alignment deflector 30 is also provided to realign the beam after it has passed through blanker 25.
Following realignment, the beam enters a final lens 35 which focuses it onto target 65, the object point of the final lens 35 being the beam cross-over at the center of the blanker. Element 40 is located within its final lens 35, serving as a third alignment deflector and second stigmator. The next element down the column is a dynamic focus coil 45. This serves as a fine focus for beam 60 as it is being deflected to the appropriate location on the target via high speed deflector 50 and a precision deflector 55. Element 70 is an electron scintillator which is connected to a light, pipe and multiplier 75, which are used to accurately monitor the device as it is writing.
FIG. 2, also from Kelly, shows an enlarged view of the blanker 25. This blanker has two characteristic U-shaped plates; the rear plate is only partially visible in this view. The deflector plates 260 are spaced apart by 0.10 inch, each having the general shape of a U and being symmetric top to bottom. Electromagnetic energy (the blanking signal) enters this structure through the two leads at point A, traverses a transition region, travels a length of the U and the top half is reflected around the corner, travels back the length of the U on the bottom half, traverses another transition region and exits the leads at point B. The plates have a constant thickness except in the transition regions 270. Other illustrated dimensions are as indicated in Kelly. There is a knife-edge 280 suspended between the plates by a knife-edge support 290. This is the beam blanking aperture. The electron beam is seen passing between the plates and through the aperture 280. Also provided are mounting holes 271. Angles A1 and A2 are designed to optimize performance.
There is a well-known technical problem in conjunction with such blankers. The RF (radio frequency) blanking signal typically propagates near the velocity of light. This velocity is significantly faster than that of the electron beam. It is to be understood that electron beams have no particular characteristic velocity; the velocity of propagation is dependent on the amount of accelerating energy to which they are subject. Typically, even in a very high voltage electron beam column, electron velocity is no more than 0.4 times the velocity of light. In any case, there is typically a mis-match in velocities due to the electron beam propagating much slower than the blanking signal. This leads to non-ideal beam deflections which in turn cause writing anomalies. This problem is recognized in Kelly and compensated for by providing an electrical path length, which is effectively a delay line, incorporated into the blanker. The transit time of the electromagnetic wave of the blanking signal on this path corresponds approximately to the time for an electron in the beam to transverse the width of the top half of the plates. Thus, an electron in the beam entering the blanker will be subject to substantially the same electric field above the beam cross-over as it is below it but delayed in time.
Thus, Kelly uses the delay line approach to match electron beam velocity with the blanking signal propagation velocity. It is to be understood that any mis-match in these velocities results in focusing problems and hence a less precise image being formed on the target.
Other types of electron beam blankers are shown in Gesley et al., U.S. Pat. No. 5,412,281 and Gesley, U.S. Pat. No. 5,276,330 both also incorporated herein by reference and having structures somewhat similar to those of Kelly.
Feuerbaum, U.S. Pat. No. 4,169,229 also incorporated herein by reference, recognizes the same delay issue but solves it differently. Instead of a U-shaped delay line extending along the deflector plate, Feuerbaum uses a meandering conductor as a delay line and also as the deflector plate. This is shown in present FIG. 3, identical to FIG. 2 of Feuerbaum and having the same reference numbers. Here instead of U shaped deflector plates, the plates are planar and rectangular. Only a single plate of the pair is shown in FIG. 3; the other plate is not shown for simplicity.
The electron beam 15 is shown as a dotted line transiting the plate. The plate includes the interdigital structure 51 which includes a meander shaped conductor track 52 and a screening or shielding conductor track 53. Track 53 is grounded. The conductor track 52 has a (schematically indicated) input connection 521 and an output connection 522 likewise grounded via a terminal resistance 523. A high frequency electromagnetic wave (the blanking signal) enters via the input terminal 521 of the conductor track structure 52 and travels along the meander shape path so as to progress in the direction shown by the arrow 15, delayed in accordance with the extension of the path. The dimensioning of the meander shaped track is determined as a function of the speed of the electron beam which in turn is determined by its accelerating voltage. Kelly indicates that the interdigital structure 51 may be applied to a plate of dielectric material.
Hence, blanking the beam is an electric deflection of the beam caused by two parallel plates that extend parallel to the beam for a distance L and on either side of the beam. A difficulty arises when the blanking signal applied to the plates changes quickly with respect to the electron beam transit time along the length L. Then the fact that the wave (signal) velocity does not match the electron beam velocity causes non-negligible distortions of the electron beam deflection. Feuerbaum addresses this problem by his meandering delay line. The resulting disadvantage is that the beam is only deflected at discrete points of the line, resulting in not much deflection per volt of signal (the deflection sensitivity is low). A secondary disadvantage is that the structure is complex with many bends in the meandering delay line, which may result in wave energy being partially reflected at the bends and thus distorting the waveform.
The Feuerbaum and Kelly approaches thus have been found by the present inventors to have significant drawbacks. Both have problems with high frequency blanking signals in terms of RF propagation and interference. Also, in both cases, the electron beam is subject to rather uneven influences due to the unusual (non-linear) path taken by the blanking signal. Thus, at any one instant, portions of the beam may be subject to a blanking signal of different intensity depending on the position of the blanking signal relative to the beam. That is, the blanking signal at different times is closer or further away from the beam when following its delay path in both Kelly and Feuerbaum.
Therefore, a better match between electron beam propagation and blanking signal propagation would be desirable, especially for high frequency blanking. It is to be understood that over the years the frequency of beam blanking has increased as the throughput speed of electron beam lithography has increased.
In accordance with this disclosure, an improved delay is provided for an electron beam blanker. As in Feuerbaum, the blanker has a deflector assembly including two parallel deflector plates. However, instead of a meandering conductive path, the delay is provided by propagating the blanking signal between an upper deflection region and a lower deflection region of each deflector plate through a connection region including a substrate having a relatively low dielectric constant. Thus the blanking signal propagates relatively slowly across the connection region between the upper and lower deflection regions, thereby providing the needed delay. The delay path in one embodiment, instead of meandering, is a curve. This use of a dielectric material to slow down propagation of the electromagnetic blanking signal to match the electron beam propagation is advantageous because it reduces RF interference. This results in less reflection and larger deflections per applied volt because the electron beam is continuously deflected rather than discretely deflected as in the Feuerbaum approach by each individual meander turn. Also, it provides continuous in-phase deflection.
In this disclosure, the blanking signal wave is slowed down by a high dielectric-constant material positioned behind, in one embodiment, the deflector plates. When the wave matches the electron beam velocity, the deflector structure can be simply two parallel plates. Since there are two deflection regions with the blanking aperture therebetween reduces the electron beam motion at the target during blanking.
One deflector plate structure in accordance with this disclosure includes a conductive waveguide, which is, e.g., a thin film of gold, formed on a dielectric substrate. At the two deflection regions, respectively at the upper and lower portion of the deflector plate, the underlying substrate has a high dielectric constant. The intervening connection region lying between the upper and lower deflection regions, being formed of a suitable dielectric constant material, functions as a delay line so that the blanking signal arrives at the upper and lower deflection regions in synchronization (in phase) with passage of the slower propagating electron beam. The goal is that when the electron beam is deflected equally by the two deflection regions, the electron beam appears to rock about the blanking aperture. Since the downstream lens, called the objective lens, as described above is focused on this point, the rocking does not cause a shifting of the beam impingement position on the target. This results in optimum exposure definition. As in the prior art, the blanking stop aperture is located halfway between the upper and lower deflection regions, approximately at the rocking point.
Thus the blanking signal electromagnetic wave propagating between the upper and lower deflection regions has a velocity controlled by the preselected configuration of the deflector plate in terms of the dielectric constant of the plate substrate (which is material dependent), the width and length of the conductive waveguide between the upper and lower deflection regions, and the overall geometry of the deflection plate. Thus in order to configure any particular deflector blanking structure, the blanking signal wave velocity is found by theoretical considerations well known to one knowledgeable in the art and the particular configuration of the deflector blanking plates is chosen accordingly.